For several years, my laboratory has been engaged in studying the mechanisms of action of the glucocorticoid class of steroid hormones. Recently, we have found that a variety of cell types contain an endogenous, small Mr, heat-stable factor that, like molybdate, both stabilizes unoccupied glucocorticoid receptors and inhibits transformation of steroid-bound receptors to a state that binds to nuclei and DNA. We have developed a system for assaying the factor in a quanitative manner and have purified it approximately 200-fold (Leach et al. J. Biol. Chem., in press). As a variety of phosphatase inhibitors can stabilize glucocorticoid receptors, we have asked if the endogenous factor is an inhibitor of protein phosphatase activity and we have found that the 200-fold purified factor preparation inhibits phosphorylase phosphatase (protein phosphatase C). The physical properties of this factor are similar to an inhibitor of phosphoprotein phosphatase activity recently reported by Larner and his coworkers (Cheng et al. Diabetes 29:659, 1980). In this proposal I intend: 1) to purify the heat-stable factor; 2) to determine its composition and structure; 3) to determine whether the factor stabilizes other steroid receptors; 4) to determine the mechanism of glucocorticoid receptor stabilization; 5) to determine the mechanism by which the factor inhibits phosphorylase phosphatase acitvity; 6) to rest the proposal that the heat-stable factor modulates ther ate of dephosphorylation of a variety of proteins. It is posible that this factor represents a novel regulatory substance. The purpose of the work in this proposal is to achieve a more detailed understanding of the factors that affect steroid binding activity and the way in which they relate to more general mechanisms by which protein phosphorylation may be regulated. The steroid hormones are used in the therapy of a wide variety of diseases and understanding the mechanisms by which their receptors may be regulated constitutes a fundamental problem in molecular endocrinology.